It is unreasonable to assume that anyone had extraordinary luck when they picked up the Saturn's driver at the accident site, dirtbag or tandem driver.
If we assume that the Saturn's driver was Maura Murray:
It is, instead, reasonable to assume that there was no luck involved at all: that she watched Butch Atwood, and when out of sight, ran down the road - or traveled down a pathway that was not being watched (Old Peter's).
In other words, I believe it is more likely that Maura Murray took steps to avoid being seen - rather then a bad person crossing her path by chance.
Even if a bad guy crossed her path by chance, the "Law of Noisy Abductions" tells us she would have screamed: https://www.reddit.com/r/MauraMurrayEvidence/comments/4xxmgp/the_law_of_noisy_abductions_opinion/
If one assumes the encounter was not by chance, one can only assume that there was a tangent driver.
However, if Maura Murray was the Saturn's driver, it does not make any sense for there to have been a tangent driver in the first place,see: https://www.reddit.com/r/MauraMurrayEvidence/comments/4xxqyl/the_first_law_of_tangential_motion_opinion/
If Maura Murray was not present, i.e. she did not drive the Saturn, and was not a passenger, then:
If there was a tangent driver, the Saturn's driver would have got into the tangent vehicle as soon as it was practical: when Butch Atwood was out of sight.
However, the facts discussed about the time Maura was last seen at Umass, and the discussion of travel time to New Hampshire weigh heavily against the proposition that Maura Murray was not present at the accident site. There simply is too little time unaccounted for to allow for Maura to be harmed and then for the spontaneous drive to take place. Therefore she was there at the accident site.
The law of 91 (or parallel) here: https://www.reddit.com/r/MauraMurrayEvidence/comments/4yagz8/the_law_of_91_or_parallel_opinion/?st=isqhruu2&sh=6cd731ce
So, all that remains is the possibility that she was a passenger in the Saturn, but not the Saturn's driver.